US authorities had 15 contacts with the Columbine teenagers up to two years before their school killing rampage, the Colorado attorney general says.

Ken Salazar said he was investigating whether the Jefferson County sheriff's office tried to cover up what it knew about the school massacre.

But he did not blame the office for missing warning signs about killers Eric Harris, 18, and Dylan Klebold, 17.

They shot dead 12 fellow students and a teacher before killing themselves.

The April 1999 attack was the deadliest school shooting in America.

Mr Salazar told a news conference in Lakewood - where some relatives of the dead students were also present - that he found no evidence of negligence by the officials.

But he stressed that the inquiry was not complete since he had been asked to check contacts between the killers and investigators beginning in 1997.

Missing file

The attorney general said his investigation looked at how the sheriff's office reacted to complaints about Harris and Klebold.

The killers gunned down their fellow students with these shotguns

In a statement, he said the contacts "initially involved incidents such as snowball-throwing and a phone-prank call, but progressed to threats via the internet and a van break-in".

Mr Salazar also said authorities were still trying to find a file detailing a search warrant affidavit for Harris' home after a pipe bomb was found along a bicycle path in 1998.

"The attorney general's office intends to pursue the whereabouts of that file and we will issue a supplemental report," the statement said.

"In the end, none of the many efforts to open up the Columbine records, including today's activity, will mean much beyond passing curiosity if we cannot learn from this tragedy."

Evidence shown

Some of the relatives of the killed students said the report did not provide them the answers they were seeking.

"This raise more questions than it answers," Dawn Anna, whose daughter Lauren Townsend was killed in the attack, was quoted by the Associated Press news agency as saying.

"I would disagree that there was no negligence," she added.

Earlier on Thursday, the members of the public were finally allowed to view a display of the evidence collected at the crime scene - the murder weapons, the chairs and tables where the victims were shot.

Authorities also released two videos - a scene in a park across the street from the school on the day of the attack, and a compilation of videos made by the killers.